Eurovan Westfalia Weekender w/flat fiberglass...

You'll have to bolt artificial rain gutter brackets to the sides of the roof. Tracks along top are another option. You'd use rain gutter towers with the brackets, and tracker II towers with tracks. And I don't recommend getting the aero bars. Just get the regular square ones. They are half the price, and you aren't limited in your accessory options as you are with the aero bars.